1. Field of the Invention
The present invention relates to a differential phase detection device having an improved structure and a tracking error signal detection apparatus using the same.
2. Description of the Related Art
Generally, as shown in FIGS. 1 and 2, a tracking error signal detection apparatus using a differential phase detection (DPD) device includes a four-division photodetector 10 for receiving light reflected from a recording medium, and differential phase detection devices 20 and 30 for receiving four detection signals a, b, c, and d output from the four-division photodetector 10, generating a plurality of phase difference signals P1, P2, P3, P4, P5, and P6, and performing an operation on the plurality of phase difference signals to detect a tracking error signal TES′ and TES″.
The four-division photodetector 10 is divided into two parts, one part in a direction (R′ direction) corresponding to a radial direction of the recording medium and another part in a direction (T′ direction) corresponding to a tangential direction of the recording medium. First through fourth division plates A, B, C, and D of the four-division photodetector 10 are arranged counterclockwise and generate the first through fourth detection signals a, b, c and d, respectively.
Referring to FIG. 1, the differential phase detection device 20, as an example of conventional one, includes four capacitors 21, four equalizers 22, four slicers 23, four phase shifters 24, two phase difference detectors 25 and 27 and a matrix circuit 29. The capacitors 21 perform AC coupling on the respective first through fourth detection signals a, b, c, and d, thereby removing DC components. Each equalizer 22 amplifies the high-frequency component of a signal input via the corresponding capacitor 21. Each slicer 23 digitizes the signal amplified by the corresponding equalizer 22. Each phase shifter 24 shifts the phase of the digitized signal to control an offset or a balance of a final output. The phase difference detector 25 detects a phase difference between the digitized signals corresponding to the first and second detection signals a and b, which are input from corresponding two phase shifters 24, and outputs first and second phase difference signals p1 and p2. The other phase difference detector 27 detects a phase difference between the digitized signals corresponding to the third and fourth detection signals c and d, which are input from corresponding two phase shifters 24, and outputs third and fourth phase difference signals p3 and p4. The matrix circuit receives the first through fourth phase difference signals p1, p2, p3, and p4 and performs an operation on the signals, thereby outputting the tracking error signal TES′. The tracking error signal TES′ is a differential signal between a sum p1+p3 of the first and third phase difference signals p1 and p3 and a sum p2+p4 of the second and fourth phase difference signals p2 and p4. Because the conventional differential phase detection device 20 includes the two phase difference detectors 25 and 27 to detect the phase difference signals, the entire volume of the differential phase detection device 20 is large, and an output signal can be degraded due to the difference between the two phase difference detectors 25 and 27 in a gain characteristic.
Referring to FIG. 2, the differential phase detection device 30, as another example of a conventional device, has a structure that overcomes the problems of the differential phase detection device 20 of FIG. 1. The differential phase detection device 30 includes four capacitors 31, four delay units 32, two equalizers 33a and 33b, two slicers 34a and 34b, two phase shifters 35a and 35b, a phase difference detector 37 and a matrix circuit 39. In FIGS. 1 and 2, the same reference numerals denote same elements having the same functions.
The capacitors 31 perform AC coupling on respective first through fourth detection signals a, b, c and d received from the four-division photodetector 10, thereby removing DC components. The delay units 32 time-delay the first through fourth detection signals a, b, c, and d received from the respective capacitors 31. The delay units 32 are provided for compensating for an offset of a final output, that is, a tracking error signal TES″, when a shift of an objective lens of an optical pickup occurs, or when a depth of a pit recorded on an optical disc deviates from a specified value. The delay units 32 relatively delay the first and second detection signals a and b output from the preceding first and second division plates A and B, respectively, in a T′ direction or the third and fourth detection signals c and d output from the succeeding third and fourth division plates C and D, respectively. The delay units 32 appropriately delay the first detection signal a and/or the third detection signal c such that a delay value applied to the first detection signal a becomes positive or negative with respect to the third detection signal c. Likely, the delay units 32 appropriately delay the second detection signal b and/or the fourth detection signal d such that a delay value applied to the second detection signal b becomes positive or negative with respect to the fourth detection signal d.
The first and third detection signals a and c output from the delay units 32 are summed and equalized by the equalizer 33a. The second and fourth detection signals b and d output from the delay units 32 are summed and equalized by the equalizer 33b. The slicers 34a and 34b digitize the amplified sum signals from the equalizers 33a and 33b, respectively. The phase shifters 35a and 35b shift phases of the respective digitized sum signals to control an offset or a balance of a final output. The phase difference detector 37 detects a phase difference between sum signals received from the respective phase shifters 35a and 35b and outputs two phase difference signals p5 and p6. The matrix circuit 39 performs a differential operation on the two phase difference signals p5 and p6 received from the phase difference detector 37 to output the tracking error signal TES″.
Because the differential phase detection device 30 of FIG. 2 includes one phase difference detector 37, a problem of a gain error between the two phase difference detectors 25 and 27 occurring in the device of FIG. 1 does not occur. However, in order to realize a structure using the one phase difference detector 37, the differential phase detection device 30 of FIG. 2 employs the four delay units 32 to compensate for the phase difference between the first and third detection signals a and c and the phase difference between the second and fourth detection signals b and d. Accordingly, the total volume of the differential phase detection device 30 is large due to a large volume of the block of the delays 32, and a large amount of power is required to operate the delays 32. In addition, because the delay values of the delay units 32 are related to the frequency of a reproduced signal, a unit for selecting a delay value appropriate for a multiple speed at which data will be reproduced from an optical disc is required. Consequently, the differential phase detection device 30 of FIG. 2 has a larger volume than the differential phase detection device 20 of FIG. 1 and consumes a larger amount of power.